Pulled in from https://github.com/adobe-fonts/source-serif-pro/

See https://bugzilla.mozilla.org/show_bug.cgi?id=1545317

Pulled in from https://github.com/adobe-fonts/source-code-pro/

From https://github.com/mozilla/Fira

Stabilize futures_api

cc rust-lang#59725.
Based on rust-lang#59733 and rust-lang#59119 -- only the last two commits here are relevant.

r? @withoutboats , @oli-obk for the introduction of `rustc_allow_const_fn_ptr`.

Warn on unused results for operation methods on nums

From a suggestion by @llogiq

Adds a `#[must_use]` attribute to operation methods on integers that take self by value as the first operand and another value as the second. It makes it clear that these methods return the result of the operation instead of mutating `self`, which is the source of a rather embarrassing bug I had in a codebase of mine recently...

As an example:

```rust
struct Int {
   value: i64,
}

impl Int {
    fn add(&mut self, other: i64) {
        self.value.wrapping_add(other);
    }
}
```

Will produce a warning like:

```
warning: unused return value of `core::num::<impl i64>::wrapping_add` that must be used
 --> src/main.rs:7:7
  |
7 |       self.value.wrapping_add(other);
  |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default
  = note: this returns the result of the operation, without modifying the original
```

If this is something we're on board with, we could do something similar for `f32` and `f64` too. There are probably other methods on integers that make sense.

…dreavus

Update fonts used by rustdoc

Our version of Source Serif Pro is pretty old and is causing issues on Linux, see https://bugzilla.mozilla.org/show_bug.cgi?id=1545317 . I took this opportunity to update all of the fonts we use.

r? @steveklabnik @QuietMisdreavus

…ine, r=kennytm

Warn when ignore-tidy-linelength is present, but no lines are too long

It's easy for a `// ignore-tidy-linelength` to be added when there is a genuine need to ignore a file's line length, but then after refactoring the need is gone, but the tidy directive is not removed. This means that in the future, further editing may accidentally add unnecessarily long lines. This change forces `// ignore-tidy-linelength` to be used exactly when necessary, to make sure such changes are intentional.

Promote rust comments to rustdoc

Add f16c target_feature

This is requires for Intel 16-bit half-precision float intrinsics: https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=fp16&expand=1769 - see companion stdsimd PR: rust-lang/stdarch#737.

LLVM, Wikipedia CPUID page, and the Intel Dev Manual all call this CPUID feature "F16C", but the Intel intrinsics guide calls this "FP16C" - this is probably a bug in the intrinsics guide which we are tracking here: rust-lang/stdarch#738

r? @alexcrichton